Effect of the Local Microenvironment on Survival and Thermal Inactivation of Salmonella in Low- and Intermediate-Moisture Multi-Ingredient Foods

Multi-ingredient foods having low- or intermediate-moisture characteristics may pose a special challenge to process design and validation. Ingredients of these foods can create local microenvironments that may have a distinct impact on pathogen survival and processing requirements. In this study, two model systems, each consisting of 80% commercial peanut butter (P) and 20% nonfat dry milk powder (M), were formulated to be identical in composition, but different in the source of the Salmonella contamination as originating in either the ingredient P or M. Immediately after inoculation, Salmonella showed a 2.0-log reduction when M was the contaminated ingredient compared with a 0.6-log reduction when P was the contaminated ingredient. This pattern of survival was consistent with the single-ingredient control containing only M (2.5-log reduction) or only P (0.7-log reduction), suggesting that the immediate proximity of cells is determined by the contaminated ingredient in the model system. After 5 weeks of storage, the survival rates of Salmonella in the two systems remained different, i.e. a 4- and 2-log reduction resulted in the system with M or P as the contaminated ingredient, respectively. Furthermore, thermal inactivation efficacies also differed significantly between the two systems. Fourier transform infrared spectroscopy demonstrated the nonhomogeneous distribution of water, lipid, and protein, indicating that varied local microenvironments were present and likely affected the behavior of the pathogen. The impact of the microenvironment on inactivation and survival of Salmonella was further confirmed in a butter cookie formulation in which Salmonella was inoculated via four different ingredients. This study shows that the local microenvironment in low- and intermediate-moisture foods affects Salmonella survival and thermal inactivation. The ingredient source of the contamination should be taken into account for process design and validation to ensure the safety of the product.